Veljko Samardzic ME-215 Engineering Materials and Processes
Fundamentals of Metal Forming
Chapter 15
Veljko Samardzic ME-215 Engineering Materials and Processes
15.1 Introduction
• Deformation processes have been designed
to exploit the plasticity of engineering
materials
• Plasticity is the ability of a material to flow
as a solid without deterioration of properties
• Deformation processes require a large
amount of force
• Processes include bulk flow, simple
shearing, or compound bending
Veljko Samardzic ME-215 Engineering Materials and Processes
States of Stress
Veljko Samardzic ME-215 Engineering Materials and Processes
15.2 Forming Processes:
Independent Variables • Forming processes consist of independent and
dependent variables
• Independent variables are the aspects of the processes that the engineer or operator has direct control – Starting material
– Starting geometry of the workpiece
– Tool or die geometry
– Lubrication
– Starting temperature
– Speed of operation
– Amount of deformation
Veljko Samardzic ME-215 Engineering Materials and Processes
Forming Operations
Veljko Samardzic ME-215 Engineering Materials and Processes
Forming Operations
Veljko Samardzic ME-215 Engineering Materials and Processes
Forming Operations
Veljko Samardzic ME-215 Engineering Materials and Processes
15.3 Dependent Variables
• Dependent variables are those that are
determined by the independent variable
selection
– Force or power requirements
– Material properties of the product
– Exit or final temperature
– Surface finish and precision
– Nature of the material flow
Veljko Samardzic ME-215 Engineering Materials and Processes
15.4 Independent-Dependent
Relationships
• Independent variables- control is direct and
immediate
• Dependent variables- control is entirely
indirect
– Determined by the process
– If a dependent variable needs to be controlled,
the designer must select the proper independent
variable that changes the dependent variable
Veljko Samardzic ME-215 Engineering Materials and Processes
Independent-Dependent
Relationships • Information on the
interdependence of
independent and
dependent variables
can be learned in three
ways
– Experience
– Experiment
– Process modeling
Figure 15-1 Schematic representation of a
metalforming system showing independent
variables, dependent variables, and the various
means of linking the two.
Veljko Samardzic ME-215 Engineering Materials and Processes
15.5 Process Modeling
• Simulations are created using finite element
modeling
• Models can predict how a material will
respond to a rolling process, fill a forging
die, flow through an extrusion die, or
solidify in a casting
• Heat treatments can be simulation
• Costly trial and error development cycles
can be eliminated
Veljko Samardzic ME-215 Engineering Materials and Processes
15.6 General Parameters
• Material being deformed must be
characterized
– Strength or resistance for deformation
– Conditions at different temperatures
– Formability limits
– Reaction to lubricants
• Speed of deformation and its effects
• Speed-sensitive materials- more energy is
required to produce the same results
Veljko Samardzic ME-215 Engineering Materials and Processes
15.7 Friction and Lubrication Under
Metalworking Conditions • High forces and pressures are required to deform a
material
• For some processes, 50% of the energy is spent in overcoming friction
• Changes in lubrication can alter material flow, create or eliminate defects, alter surface finish and dimensional precision, and modify product properties
• Production rates, tool design, tool wear, and process optimization depend on the ability to determine and control friction
Veljko Samardzic ME-215 Engineering Materials and Processes
Friction Conditions
• Metalforming friction
differs from the friction
encountered in mechanical
devices
• For light, elastic loads,
friction is proportional to
the applied pressure
– μ is the coefficient of
friction
• At high pressures, friction
is related to the strength of
the weaker material
Figure 15-2 The effect of contact pressure
on the frictional resistance between two
surfaces.
Veljko Samardzic ME-215 Engineering Materials and Processes
Friction
• Friction is resistance to sliding along an
interface
• Resistance can be attributed to:
– Abrasion
– Adhesion
• Resistance is proportional to the strength of
the weaker material and the contact area
Veljko Samardzic ME-215 Engineering Materials and Processes
Surface Deterioration
• Surface wear is related to friction
• Wear on the workpiece is not objectionable,
but wear on the tooling is
• Tooling wear is economically costly and
can impact dimensional precision
• Tolerance control can be lost
• Tool wear can impact the surface finish
Veljko Samardzic ME-215 Engineering Materials and Processes
Lubrication
• Key to success in many metalforming
operations
• Primarily selected to reduce friction and
tool wear, but may be used as a thermal
barrier, coolant, or corrosion retardant
• Other factors
– Ease of removal, lack of toxicity, odor,
flammability, reactivity, temperature, velocity,
wetting characteristics
Veljko Samardzic ME-215 Engineering Materials and Processes
15.8 Temperature Concerns
• Workpiece temperature can be one of the most important process variables
• In general, an increase in temperature is related to a decrease in strength, increase in ductility, and decrease in the rate of strain hardening
• Hot working
• Cold working
• Warm working
Veljko Samardzic ME-215 Engineering Materials and Processes
Hot Working
• Plastic deformation of metals at a
temperature above the recrystallization
temperature
• Temperature varies greatly with material
• Recrystallization removes the effects of
strain hardening
• Hot working may produce undesirable
reactions from the metal and its
surroundings
Veljko Samardzic ME-215 Engineering Materials and Processes
Structure and Property Modification
by Hot Working
• The size of grains upon cooling is not
typically uniform
• Undesirable grain shapes can be common
(such as columnar grains)
• Recrystallization is followed by:
– grain growth
– additional deformation and recrystallization
– drop in temperature that will terminate
diffusion and freeze the recrystallized structure
Veljko Samardzic ME-215 Engineering Materials and Processes
Hot Working
• Engineering properties
can be improved
through reorienting
inclusion or impurities
• During plastic
deformation,
impurities tend to flow
along with the base
metal or fraction into
rows of fragments
Figure 15-4 Flow structure of a hot-forged gear
blank. Note how flow is parallel to all critical
surfaces. (Courtesy of Bethlehem Steel
Corporation, Bethlehem, PA.)
Figure 15-3 Cross section of a 4-in.-diameter
case copper bar polished and etched to show the
as-cast grain structure.
Veljko Samardzic ME-215 Engineering Materials and Processes
Temperature Variations in Hot
Working • Success or failure of a hot
deformation process often depends on the ability to control temperatures
• Over 90% of the energy imparted to a deforming workpiece is converted to heat
• Nonuniform temperatures may be produced and may result in cracking
• Thin sections cool faster than thick sections
Figure 15-5 Schematic comparison of the
grain flow in a machined thread (a) and a rolled
thread (b). The rolling operation further
deforms the axial structure produced by the
previous wire- or rod-forming operations, while
machining simply cuts through it.
Veljko Samardzic ME-215 Engineering Materials and Processes
Cold Working
• Plastic deformation below the recrystallization temperature
• Advantages as compared to hot working
– No heating required
– Better surface finish
– Superior dimensional control
– Better reproducibility
– Strength, fatigue, and wear are improved
– Directional properties can be imparted
– Contamination is minimized
Veljko Samardzic ME-215 Engineering Materials and Processes
Disadvantages of Cold Working
• Higher forces are required to initiate and complete
the deformation
• Heavier and more powerful equipment and
stronger tooling are required
• Less ductility is available
• Metal surfaces must be clean and scale-free
• Intermediate anneals may be required
• Imparted directional properties can be detrimental
• Undesirable residual stresses may be produced
Veljko Samardzic ME-215 Engineering Materials and Processes
Metal Properties and Cold
Working • Two features that are significant in selecting a material for cold
working are
– Magnitude of the yield-point stress
– Extent of the strain region from yield stress to fracture
• Springback should also be considered when selecting a material
Figure 15-6 Use of true stress-true strain diagram to assess
the suitability of two metals for cold working.
Veljko Samardzic ME-215 Engineering Materials and Processes
Initial and Final Properties in a
Cold-Working Process • Quality of the starting
material is important to the success or failure of the cold-working process
• The starting material should be clean and free of oxide or scale that might cause abrasion to the dies or rolls
Figure 15-7 (Below) Stress-strain
curve for a low-carbon steel
showing the commonly observed
yield-point runout; (Right) Luders
bands or stretcher strains that
form when this material is
stretched to an amount less than
the yield-point runout.
Veljko Samardzic ME-215 Engineering Materials and Processes
Additional Effects of Cold
Working • Annealing heat treatments
may be performed prior or
at intermediate intervals to
cold working
• Heat treatments allows
additional cold working
and deformation processes
• Cold working produces a
structure where properties
vary with direction,
anisotropy Figure 15-8 Mechanical properties of pure copper
as a function of the amount of cold work
(expressed in percent).
Veljko Samardzic ME-215 Engineering Materials and Processes
Warm Forming
• Deformations produced at temperatures intermediate to cold and hot working
• Advantages
– Reduced loads on the tooling and equipment
– Increased material ductility
– Possible reduction in the number of anneals
– Less scaling and decarburization
– Better dimensional precision and smoother surfaces than hot working
– Used for processes such as forging and extrusion
Veljko Samardzic ME-215 Engineering Materials and Processes
Isothermal Forming
• Deformation that occurs
under constant
temperature
• Dies and tooling are
heated to the same
temperature as the
workpiece
• Eliminates cracking from
nonuniform surface
temperatures
• Inert atmospheres may be
used
Figure 15-9 Yield strength of various materials (as
indicated by pressure required to forge a standard
specimen) as a function of temperature. Materials
with steep curves may require isothermal forming.
(From “A Study of Forging Variables,” ML-TDR-64-
95, March 1964; courtesy of Battelle Columbus
Laboratories, Columbus, OH.)